Structural silicon nitride materials containing rare earth oxides

ABSTRACT

A ceramic composition suitable for use as a high-temperature structural material, particularly for use in apparatus exposed to oxidizing atmospheres at temperatures of 400 to 1600 DEG  C., is found within the triangular area ABCA of the Si3N4-SiO2-M2O3 ternary diagram depicted in FIG. 1. M is selected from the group of Yb, Dy, Er, Sc, and alloys having Yb, Y, Er, or Dy as one component and Sc, Al, Cr, Ti, (Mg +Zr) or (Ni+Zr) as a second component, said alloy having an effective ionic radius less than 0.89 A.

BACKGROUND OF THE INVENTION

This invention was made in the course of, or under, a contract with theU.S. Department of Energy.

It relates to high-temperature corrosion-resistant materials and morespecifically to hot pressed Si₃ N₄ ceramics containing oxidedensification aids. Densification aids in the Si₃ N₄ system functionduring hot pressing via the formation of a liquid phase which dissolvesSi₃ N₄ and acts as a transport medium during the pressing operation,resulting in a reduction in porosity. During cool-down Si₃ N₄ and otherphases precipitate from the liquid phase. In the prior art a number ofcompounds have been suggested as densification aids for Si₃ N₄, such asCeO₂, MgO, Mg₃ N₂, Al₂ O₃, and a number of rare earth oxides. A numberof possible densification additives for Si₃ N₄ were screened asdescribed by I. C. Huseby and Petzow in the article "Influence ofVarious Densifying Additives on Hot Pressed Si₃ N₄," Powder MetallurgyInternational, Vol. VI, February 1974, pp. 17-19. Of the materialstested Ce₂ O₃, CeO₂, MgO, and La₂ O₃ were found to be the most effectiveas additives, while Gd₂ O₃, Yb₂ O₃, and BcO were said to be of mediumeffectiveness. A number of other additives were said to be of nobenefit.

In U.S. Pat. No. 4,102,698 to Frederick F. Lange, et al., for "SiliconNitride Compositions in the Si₃ N₄ -Y₂ O₃ -SiO₂ System," issued July 25,1978, it was reported that Y₂ O₃, when used as a densification additivein a specified area of the Si₃ N₄ -SiO₂ -Y₂ O₃ ternary diagram resultsin enhanced corrosion resistance with respect to other Si₃ N₄compositions containing Y₂ O₃. This corrosion resistance was attributedto the elimination of detrimental secondary phases which are readilyoxidized. The ceramics fabricated in the Si₃ N₄, Si₂ N₂ O, and Y₂ Si₂ O₇phase field were found to contain no unstable phases.

SUMMARY OF THE INVENTION

It is an object of this invention to provide Si₃ N₄ -based ceramiccompositions suitable for use in high temperature oxidizingenvironments, such as gas turbine, etc., which have improved oxidationresistance in combination with comparable or improved mechanicalstrength, relative to prior art compositions containing SiO₂ and Y₂ O₃.

It is a further object to provide an improvement in articles ofmanufacture having a component intended for use in an oxidizingatmosphere at a temperature in excess of 400° C.

These and other objects are achieved according to this invention in aceramic composition suitable for use as a high-temperature structuralmaterial, said composition being within the triangular area defined bythe points ABCA of the Si₃ N₄ -SiO₂ -M₂ O₃ ternary diagram depicted inFIG. 1, M being selected from the group consisting of Yb, Dy, Er, Sc,and alloys having Yb, Dy, Er or Y as a first component, and Sc, Al, Cr,Ti, (Mg+Zr) or (Ni+Zr) as a second component said alloy having aneffective ionic radius less than 0.89 A.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a ternary Si₃ N₄ -SiO₂ -M₂ O₃ diagram. Area ABCA representsthe phase field in which corrosion resistance is achieved according tothe present invention.

FIG. 2 is a graph depicting the variation of flexural strengths andincipient melting points of Si₃ N₄ based compositions as a function ofthe ionic radius of the metal in candidate Y₂ O₃ and rare earth oxideadditives.

DETAILED DESCRIPTION

The oxidation resistance within the ABCA compatibility triangle in theSi₃ N₄ -SiO₂ -Y₂ O₃ system is attributable to the fact that the triangleis bounded by points Y₂ Si₂ O₇ and Si₂ N₂ O, both fully oxidizedcompositions. It is believed that the corrosion resistance of Si₃ N₄ inoxygen-containing atmospheres is due in part to the formation of apassivating SiO₂ surface upon exposure to oxygen, especially at hightemperatures. According to this invention, it has been found that Y₂ O₃is not unique in its ability to impart improved strength to Si₃ N₄ whileretaining a significant measure of corrosion resistance. It has beenfound that certain of the rare earth oxides when added to Si₃ N₄, alsodemonstrate the formation of a stable M₂ Si₂ O₇ phase during hotpressing. In some of the rare earth oxide Si₃ N₄ -SiO₂ systems, thesecondary phase formed upon cool-down is glassy. A glassy phase isbelieved to flow at high temperatures causing grain boundary sliding andslow crack growth in Si₃ N₄ -based ceramics. Several of the rare earthoxides, however, were found to result in a crystalline M₂ Si₂ O₇ phasewhich provides a significant increase in strength. Certain of these rareearth oxides which form crystalline M₂ Si₂ O₇ boundary phases in the Si₃O₄ -SiO₂ system have quite high liquid phase temperatures (temperatureof incipient melting) which permits mechanical strength retention athigh use temperatures.

Based upon a survey of eleven rare earth and yttrium oxides, it has beendiscovered that the highest strength and highest incipient meltingpoints are achieved in those Si₃ O₄ -SiO₂ -M₂ O₃ ceramic systems inwhich the M component has an ionic radius less than about 0.93 A whichis close to that of yttrium (Y⁺³ =0.89 A).

In order to determine whether additives other than Y₂ O₃ could result inthe formation of the desirable M₂ Si₂ O₇ phase and provide excellenthigh temperature mechanical properties and corrosion resistance, rareearth oxides and yttrium oxide additives were investigated for bothcrystallinity and for the incipient melting points of the lowest meltingphase. The test compositions were selected to produce 50 mole percentSi₃ N₄ --50 mole percent M₂ Si₂ O₇, and 33 mole percent Si₃ N₄ --33 molepercent Si₂ N₂ O--33 mole percent M₂ Si₂ O₇. The mixtures were ballmilled and cold pressed into pellets 3/8 inch diameter×1/4 inch thick. A1/16 inch diameter by 1/4 inch deep sight hole was radially drilled intoeach pellet. The pellets were individually inductively heated using amolybdenum susceptor in argon gas until melting occurred. Temperatureswere continuously monitored. The incipient melting temperatures aregiven in the upper portion of FIG. 2, and are the average of the initialmelting temperatures of the two compacts. In some cases only theexterior of the pellet underwent melting, and on subsequent examinationthe pellets were shown not to have melted in the region of the drilledsight hole. The upward arrows indicate these compositions. Of thespecimens heated, those prepared with Nd, Sm, Gd, Dy, and Pr had poor orvery poor crystallinity and contained glassy phases in at least onesample. Those specimens prepared with Sc, Y, La, and Ce contained M₂ Si₂O₇ phases with good crystallinity. The best crystallinity was observedin those specimens prepared with Yb and Er oxides. The compositionprepared with Yb₂ O₃ contained both Yb₂ Si₂ O₇ (major) and Yb₂ SiO₅(minor), indicating that the short duration of the test did not allowequilibrium to be established.

A number of Si₃ N₄ flexural specimens were prepared from eleven rareearth oxides and from Cr₂ O₃, NiO, and ZrO₂ which are of possibleutility as pressing aids. Mixing and particle size reduction of powderswere accomplished by ball milling them with tungsten carbide millingmedia and a liquid such as tert-butanol or methanol in polyethylenejars. The Si₃ N₄ used was from a single lot of high purity powdercontaining less than 200 ppm Ca, less than 0.6 wt.% oxygen and having anoverall purity of 99%. Hot pressing was accomplished in a conventionalclamshell hot press in a nitrogen or argon atmosphere, at a slightlypositive pressure. Pressures of 4000-5000 psi, were applied to graphitedies which were inductively heated to temperatures between 1750° and1800° C. Compaction was monitored by externally measuring ram travel.Pressing times were those required for completion (ram travel rateapproaching zero) and ranged from a minimum of 2 hours to a maximum of 4hours. The resulting discs were 2 inches in diameter by 0.3 inch thick.Test specimens were sliced from the pressed disc to provide specimens ofabout 0.250×0.125×1.25 inches. The tensile stressed surfaces were groundparallel to the tensile direction with a 325 mesh diamond wheel.Flexural testings were performed in an instron Universal test machine ata crosshead speed of 0.002 in./min. The room temperature test fixtureshad inner and outer spans measuring 0.250 inch and 0.750 inch,respectively. The Si₃ N₄ elevated temperature test fixtures had spans of0.375 inch and 0.875 inch. The rare earths oxide compositions aredepicted in Table I. All the rare earths with the exception of Scprovided sufficient liquid to allow densification under the hot pressingtemperatures and pressures used. Due to their higher incipient meltingpoints the Sc₂ O₃ -containing compositions require higher hot pressingtemperatures and pressures. As shown in Table I, the compositions Dy₂O₃, Er₂ O₃, Yb₂ O.sub. 3 provided excellent flexural strength at bothroom temperature and 1400° C. It is not presently understood why the Dy₂O₃ additive exhibited such good mechanical properties since the binderphase should have been glassy as indicated by melting tests. It isbelieved that a crystalline phase was actually produced during the hotpressing operation which was of longer duration than the above-describedmelting point experiment. The Cr₂ O₃, NiO₂, and ZrO₂ did not proveuseful as densification aids in combination with SiO₂. Each of thesesystems had free or liquid SiO₂ at the hot pressing temperature whichreacts with the Si₃ N₄ to form solid Si₂ N₂ O, providing no liquid phasesintering or densification.

                                      TABLE 1                                     __________________________________________________________________________    Compositions and Flexural Strengths of Silicon Nitride Hot-Pressed with       Metal Oxide-Silica Additions                                                                 Calculated                                                                             Percent                                                                             Flexural Strengths, ksi                         Mole Fraction  Mole Fraction                                                                          Theoretical                                                                         Room Temperature                                                                          1400° C.                     Metal                                                                             Si.sub.3 N.sub.4                                                                  M.sub.2 O.sub.3                                                                   SiO.sub.2                                                                        Si.sub.3 N.sub.4                                                                  M.sub.2 Si.sub.2 O.sub.7                                                           Density                                                                             Avg.                                                                              High                                                                              Low Avg.                                                                             High                                                                              Low                          __________________________________________________________________________    Sc  .9000                                                                             .0333                                                                             .0667                                                                            .9643                                                                             .0357                                                                              92                                                    Sc  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              85                                                    La  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              98    62  89  14  39 51  31                           La  .9000                                                                             .0333                                                                             .0667                                                                            .9743                                                                             .0357                                                                              93    92  109 84  36 49  24                           Ce  .8000                                                                              .0667*                                                                           .1333                                                                            .9230                                                                             .0770                                                                              100   50  100 17  52 53  50                           Pr  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              100   72  90  62  39 41  38                           Nd  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              100   101 107 94  51 56  42                           Sm  .8000                                                                             .0667                                                                             .3333                                                                            .9230                                                                             .0770                                                                              98    78  88  71  33 40  26                           Gd  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              100   100 108 66  42 51  38                           Dy  .8953                                                                             .0349                                                                             .0698                                                                            .9625                                                                             .0375                                                                              95    109 115 88  58 71  51                           Dy  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              100   114 134 100 66 77  55                           Er  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              100   61  94  23  56 70  43                           Yb  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              100   110 117 101 81 102 70                           Yb  .8636                                                                             .0455                                                                             .0909                                                                            .9500                                                                             .0500                                                                              100   111 122 102 68 73  66                           __________________________________________________________________________     *Calculated from equivalent CeO.sub.2 content: 2 CeO.sub.2 →           Ce.sub.2 O.sub.3 + 1/2 O.sub.2.                                          

Table 2 depicts a comparison of the flexural strengths of Si₃ N₄ -SiO₂-M₂ O₃ ceramics prepared with Yb, Dy, and Er of this invention comparedwith ceramics prepared with approximately molar equivalent amounts ofthe prior art Y₂ O₃.

                                      TABLE 2                                     __________________________________________________________________________                         Flexural Strengths (ksi)                                 Mole Fraction  Theoretical                                                                         Room Temperature                                                                          1400° C.                                                                         Percent                            Oxide                                                                             Si.sub.3 N.sub.4                                                                    M.sub.2 SiO.sub.2                                                                  Density                                                                             Avg.                                                                              High                                                                              Low Avg.                                                                             High                                                                              Low                                                                              Improvement*                       __________________________________________________________________________    Y   .9230 .0770                                                                              99    94  103 86  61 69  54                                    Yb  .9250 .0750                                                                              100   110 117 101 81 102 70 33                                 Er  .9250 .0750                                                                              100   61  94  23  56 70  43                                    Dy  .9230 .0770                                                                              100   114 134 100 66 77  55 8                                  Y   .9444 .0556                                                                              99    113 126 100 60 67  56                                    Yb  .9500 .0500                                                                              100   111 122 102 68 73  66 13                                 Er  (not tested)                                                              Dy  .9625 .0375                                                                              95    109 115 88  58 71  51                                    __________________________________________________________________________     *Relative to Y.sub.2 O.sub.3, average at 1400° C.                 

A series of oxidation tests were performed on the broken flexure barswhich had been used to measure room temperature flexural strengths. Onegroup of bars was placed in a furnace in air at 1000° C. and anothergroup was placed in another furnace at 1400° C. Both groups were heatedfor extended periods. The specimens were periodically withdrawn todetermine intermediate mass gains and weighed on a balance with anaccuracy of about ±0.0003 gram. The results are depicted in Table 3. TheKp values for material containing MgO were based upon short term tests.The Kp values for pure Si₃ N₄ were based on values reported in theliterature. It is seen from the oxidation rates for the Dy, Er, andYb-containing specimens that they are unexpectedly corrosion resistant.At 1400° C. the Dy, Er, and Yb compositions of this inventiondemonstrate oxidation rate constants which appear to be a factor of tensmaller than that of the compositions prepared with Y₂ O₃. It should benoted, however, that at these extremely low oxidation rates, the masschanges were aproaching the limits of accuracy of the weighingtechnique. Nevertheless, the compositions containing Dy, Er, and Yb haveoxidation rates which lie within the range of the intrinsic rates ofpure Si₃ N₄, about 10⁻¹⁵ to 10⁻¹³ g² cm⁻⁴ s⁻¹.

                                      TABLE 3                                     __________________________________________________________________________                            Oxidation Rate Constants, K.sub.p,*                                           g.sup.2 cm.sup.-4 s.sup.-1                                           Calculated                                                                             1000° C.                                                                      1400° C.                                Mole Fraction  Mole Fraction                                                                          599 hour                                                                             769 hour                                       Metal                                                                             Si.sub.3 N.sub.4                                                                  M.sub.2 O.sub.3                                                                   SiO.sub.2                                                                        Si.sub.3 N.sub.4                                                                  M.sub.2 Si.sub.2 O.sub.7                                                           test in air                                                                          test in air                                    __________________________________________________________________________    La  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              5.5 × 10.sup.-15                                                               7.7 × 10.sup.-14                         La  .9000                                                                             .0333                                                                             .0667                                                                            .9743                                                                             .0357                                                                              8.3 × 10.sup.-15                                                               1.3 × 10.sup.-14                         Ce  .8000                                                                             .0667**                                                                           .1333                                                                            .9230                                                                             .0770                                                                              3.2 × 10.sup.-15                                                               2.3 × 10.sup.-15                         Pr  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              1.3 × 10.sup.-16                                                               7.1 × 10.sup.-15                         Nd  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              5.2 × 10.sup.-15                                                               4.8 × 10.sup.-15                         Sm  .8000                                                                             .0667                                                                             .3333                                                                            .9230                                                                             .0770                                                                              5.7 × 10.sup.-15                                                               1.3 × 10.sup.-14                         Gd  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              6.6 × 10.sup.-15                                                               9.3 × 10.sup.-15                         Dy  .8953                                                                             .0349                                                                             .0698                                                                            .9625                                                                             .0375                                                                              2.5 × 10.sup.-15                                                               5.5 × 10.sup.-15                         Dy  .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              3.3 × 10.sup.-15                                                               7.6 × 10.sup.-16                         Er  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              5.3 × 10.sup.-15                                                               2.0 × 10.sup.-15                         Yb  .8043                                                                             .0652                                                                             .1305                                                                            .9250                                                                             .0750                                                                              6.1 × 10.sup.-16                                                               3.8 × 10.sup.-15                         Yb  .8636                                                                             .0455                                                                             .0909                                                                            .9500                                                                             .0500                                                                              6.6 × 10.sup.-16                                                               2.4 × 10.sup.-15                         Y***                                                                              .8000                                                                             .0667                                                                             .1333                                                                            .9230                                                                             .0770                                                                              7.1 × 10.sup.-15                                                               2.17 × 10.sup.-14                        Y***                                                                              .8500                                                                             .0550                                                                             .1000                                                                            .9444                                                                             .0556                                                                              6.9 × 10.sup.-16                                                               2.0 × 10.sup.-14                         Commercial Hot-Press (MgO) Si.sub.3 N.sub.4                                                                  5 × 10.sup.-12 to 1                                                     × 10.sup.-11                             Pure Si.sub.3 N.sub.4     5 × 10.sup.-17                                                               5 × 10.sup.-15 to 5                                                     × 10.sup.-14                             __________________________________________________________________________     ##STR1##                                                                      **Calculated from equivalent CeO.sub.2 content: 2CeO → Ce.sub.2        O.sub.3 + 1/2 O.sub.2.                                                        ***Tests performed on unbroken test specimens for 1000 hours.            

It is seen from FIG. 2 that the flexural strengths and incipient meltingpoints of Si₃ N₄ -Si₂ N₂ O-M₂ Si₂ O₇ system are inversely related to theionic radius of the M component. In particular, those M components whichhave a metallic ionic radius smaller than that of Y⁺³ (0.89 A) provideceramics which exhibit mechanical properties comparable to or superiorto those of the prior art materials containing Y₂ O₃. Ceramics with Si₃N₄ -SiO₂ -Sc₂ O₃ will have significantly higher flexural strengths thaneven the Yb₂ O₃ -containing ceramic.

Since the flexural strength and incipient melting points of compositionsin the Si₃ N₄ -SiO₂ -M₂ O₃ system have proven to be so heavily dependentupon the ionic radius of the M component, this invention encompassescompositions in which the M component is an alloy having a firstcomponent either trivalent Y, Yb, Dy, or Er and as a second componenttrivalent Sc, Al, Cr, Ti, or equimolar combinations of divalent andquadrivalent ions such as (Ni⁺² +Zr⁺⁴) or (Mg⁺² +Zr⁺⁴). For purposes ofthis invention, the term "alloys" refers to metals in solid solutionoxides of the formula (M'_(1-x) M"_(x))₂ O₃, where M' is Y, Yb, Dy, orEr and M" is Sc, Al, Cr, Ti, (Ni+Zr) or (Mg+Zr). Such solid solutionoxides can be prepared by thoroughly blending the M' and M" oxides priorto blending with Si₃ N₄ and SiO₂ or by other methods such ascoprecipitation of M' and M" salts from solution and subsequentcalcining to form the oxide. An alternate procedure is to prepare(M'_(1-x) M"_(x))₂ Si₂ O₇ separately, and then to blend this solidsolution pyrosilicate with Si₃ N₄ prior to hot pressing.

The addition of alloy oxides to Si₃ N₄ -SiO₂ mixtures causes theformation of the densification aid phase (M'_(1-x) M"_(x))₂ Si₂ O₇ uponhot pressing, which can have a unit cell size smaller than Y₂ Si₂ O₇ orYb₂ Si₂ O₇ and thereby demonstrate enhanced mechanical properties andhigher incipient melting temperature. In addition, the high temperaturesand pressures required to hot press Si₃ N₄ compositions containing Sc₂O₃ can be reduced by alloying Sc with metals having a slightly largerionic radius.

The compositions of this invention lie in the region ABCA of the Si₃ N₄-SiO₂ -M₂ O₃ diagram where M is selected from the group of Yb, Dy, Er,Sc, and alloys having Yb, Y, Er, or Dy as a first component and Sc, Al,Cr, Ti, (Ni+Zr), or (Mg+Zr) as the second component, said alloy havingan effective ionic radius of less than that of Y⁺³, 0.89 A. It ispreferred that the alloy have an effective ionic radius smaller than0.86 A (the ionic radius of Yb⁺³). By "effective ionic radius" it ismeant the weighted average of the ionic radii of the first and thesecond alloy components, based upon the values in Table IV.

                  TABLE IV                                                        ______________________________________                                        ion             ionic radius (A)                                              ______________________________________                                        Y.sup.+3        0.89                                                          Dy.sup.+3       0.91                                                          Yb.sup.+3       0.86                                                          Er.sup.+3       0.88                                                          Sc.sup.+3       0.73                                                          Al.sup.+3       0.51                                                          Cr.sup.+3       0.63                                                          Ti.sup.+3       0.76                                                          Mg.sup.+2       0.66                                                          Ni.sup.+2       0.69                                                          Zr.sup.+4       0.79                                                          ______________________________________                                    

As seen from tables 1, 2 and 3, an especially desirable combination ofproperties is obtained in the Si₃ N₄ -SiO₂ -Yb₂ O₃ system. The greateststrength improvement over the prior art Y₂ O₃ compositions has beendemonstrated in a composition within the molar composition rangeconsisting essentially of about 78-82% Si₃ N₄, 6-7% Yb₂ O₃, 12-15% SiO₂,however, this may not necessarily be the optimum composition for the Si₃N₄ powder used. The optimum composition for a particular lot of Si₃ N₄powder may be dependent upon its morphology and the level of impurities.It is preferred that the compositions of this invention consistessentially of the specified components; that is, excluding additionalcomponents (other than impurities ordinarily present) in amountssufficient to materially affect the high temperature strength of thecomposition.

In general the ceramic compositions of this invention in which M is Yb,Er, or Dy are most easily prepared by hot pressing a blended mixture ofSiO₂, Si₃ N₄ and M₂ O₃ powders at about 4 to 6 ksi at 1750° to 1850° C.for sufficient time, usually 1 to 4 hours to achieve 95 to 100%theoretical density, i.e., the volumetrically weighted averagetheoretical density of the Si₃ N₃, SiO₂ and M₂ O₃ components.Compositions containing Sc₂ O₃ and alloys of Sc₂ O₃ require highertemperatures and pressures due to the higher incipient melting points.

Mixtures of Si₃ N₄, SiO₂ and Sc₂ O₃ can be hot pressed in a nitrogenatmosphere at temperatures up to 1850° C. and pressures up to 15 ksi incommercially available graphite fiber reinforced dies. Alternatively,the powder mixture may be hot isostatically pressed. In hot isostaticpressing, the powder is loaded into a suitable canning material such asa welded molybdenum sheet container or a glass envelope selected to havea suitable viscosity at the hot pressing temperature. As is customary inhot isostatic pressing, the filled container is evacuated, sealed, andloaded into the pressure chamber of the press. Pressure is applied bypumps external to the chamber. A temperature of 1850° C. and pressure of20-40 ksi for 1-2 hours can be used. An advantage of the hot isostaticpressing technique is that the sealed container prevents disassociationof Si₃ N₄ at the pressing temperature and the containers can be shapedprior to pressing to reduce final machining requirements. Thepreparation of ceramics containing alloys will be illustrated by thepreparation of ceramics in the Si₃ N₄ -SiO₂ -(Sc_(1-x) Y_(x))₂ O₃system. Since Y₂ Si₂ O₇ shares a monoclinic structure with Sc₂ Si₂ O₇,complete solid solubility is expected. The same general procedures canbe used to prepare other compositions of this invention in which the Mcomponent is an alloy. It is well within the skill in the art ofceramics preparation to determine the hot pressing time, temperature,and pressure to prepare any particular composition within the scope ofthis invention.

The Si₃ N₄ -SiO₂ -(Y₀.5 Sc₀.5)₂ O₃ alloys can be prepared by mixingsuitable quantities of the oxides with the Si₃ N₄ and allowing thereactions to occur during hot pressing. For example, in order to achievea 92.5 mole% Si₃ N₄ +7.5 mole % (Y₀.5 Sc₀.5)₂ Si₂ O₇ material, 3.26 mole% Sc₂ O₃ +3.26 mole % Y₂ O₃ +13.05 mole % SiO₂ (adjusted for the oxygencontained in the Si₃ N₄) is mixed with 80.43 mole % Si₃ N₄ and hotpressed. Alternatively, a (Y₀.5 Sc₀.5)₂ O₃ material can be prepared byintimately mixing 50 mole % Sc₂ O₃ +50 50 mole % Y₂ O₃ and heating totemperatures (e.g., 1500°-1700° C.) sufficient to allow diffusion tooccur. 6.52 mole % of the resulting (Y₀.5 Sc₀.5)₂ O₃ is mixed with 13.05mole % SiO₂ and 80.43 mole % Si₃ N₄ and hot pressed. Still another wayof forming (Y₀.5 Sc₀.5)₂ O₃ is to dissolve 50 mole % ScCl₃ and 50 mole %YCl₃ in water, dry the solution and then heat in air to form the oxide.Again 6.52 mole % (Sc₀.5 Y₀.5)₂ O₃ is mixed with 13.05 mole % SiO₂ and80.43 mole % Si₃ N₄ and hot pressed. In still another method ofpreparing the Si₃ N₄ -SiO₂ -(M'_(1-x) M"_(x))₂ O₃ composition, the alloypyrosilicate can be prepared first and mixed with Si₃ N₄ powder beforehot pressing. For example, a mixture of 25 mole % Sc₂ O₃ +25 mole % Y₂O₃ +50 mole % SiO₂ can be melted at temperatures between 1700° and 1800°C., cooled and crushed. 7.5 mole % of the resulting alloy pyrosilicatepowder is mixed with 92.5 mole % Si₃ N₄ and hot pressed. In all cases,the alloy oxide or alloy pyrosilicate/Si₃ N₄ mixtures hot pressed in thesame manner as the compositions containing Si₃ N₄ Sc₂ O₃ -SiO₂.

Having demonstrated the exceptional strength and high temperatureoxidation resistance of the compositions of this invention, an aspect ofthis invention is an improved article of manufacture having a componentintended for use at a temperature of 400°-1600° C. in an oxidizingatmosphere, such as air, O₂, steam, or gases containing CO₂,NO_(x), SO₂,e.g., the combustion or gasification products of fossil fuels. Examplesof such apparatus are turbines, ceramic heat exchangers, jet aircraftcomponents, ect. The improvement in such articles of manufacture ischaracterized by a component intended for use in the high temperatureoxidizing atmosphere being a ceramic composition within the triangulararea defined by the points ABCA of the Si₃ N₄ -SiO₂ -M₂ O₃ ternarydiagram depicted in FIG. 1, and having at least 95% theoretical density,M being selected from the group consisting of Yb, Dy, Er, Sc, and alloyshaving Yb, Y, Er, or Dy as one component and Sc, Al, Cr, Ti, (Mg+Zr) or(Ni+Zr) as a second component, said alloy having an effective ionicradius less than at least 0.89 A and preferably less than 0.86 A.

What is claimed is:
 1. A ceramic composition suitable for use as ahigh-temperature structural material, said composition being within thetriangular area defined by the points ABCA of the Si₃ N₄ -SiO₂ -M₂ O₃ternary diagram depicted in FIG. 1, M being selected from the groupconsisting of alloys having Yb, Y, Er, or Dy as a first component andSc, Al, Cr, Ti, (Mg+Zr) or (Ni+Zr) as a second component, said alloyhaving an effective ionic radius less than 0.89 A.
 2. The ceramiccomposition of claim 1 in which said second component is Sc.
 3. Theceramic composition of claim 2 in which the first component is Y.
 4. Theceramic composition of claim 1 in which said alloy has an effectiveionic radius less than 0.86 A.
 5. In an article of manufacture having acomponent intended for use at a temperature of 400°-1600° C. in anoxidizing atmosphere, the improvement in which said component comprisesthe ceramic composition of claim 1 having a density at least 95 percenttheoretical density.
 6. The article of manufacture of claim 5 in whichsaid second component is Sc.
 7. The article of manufacture of claim 5 inwhich said alloy has an effective ionic radius less than 0.86 A.